The present invention relates, in general, to encoding and decoding data processing systems and, more particularly, to digital video encoder devices.
Digital video encoders convert digital data such as YCBCR and RGB into analog composite video or S-Video signals for viewing on a television screen. Usually the video data received by the digital video encoder is transferred through an ITU-R 656 interface circuit. The data received by the interface circuit is expected to have a specific number of pixels for each horizontal line that is displayed, a specific number of pixels per frame, and a stable clock. Digital video encoder performance is sensitive to clock instability that is caused by the modulation process. The chrominance quality is more affected by clock variations than luminance quality.
Digital video decoders have an input for receiving data from a Video Cassette Recorder (VCR), an antenna, a satellite receiver, a digital camera, or other electronic device. The analog signals can be received in an S-Video format having the luminance and chrominance signals separated or in a composite video format having the luminance and chrominance signals combined. The digital video decoder converts the analog input signals into digital components (e.g., YCBCR and RGB).
When the video source is a VCR, the line-to-line variations caused by mechanical limitations can be up to xc2x150 nanoseconds in the active video region and as high as xc2x15 microseconds at the end of each field. These timing variations affect the stability of the clock recovered by the video decoder. As a consequence, when the digital components are encoded by a digital encoder, the clock variations affect the chrominance subcarrier signal that is generated by the digital encoder, which directly affects the chroma quality of the viewable data. Prior art systems have attempted to generate a stable chrominance subcarrier signal by sending frequency correction information for each line of data from the digital video decoder to the digital video encoder. The frequency correction information can be transmitted via a serial interface, transmitted as ancillary data over the ITU-R 656 interface, or transmitted in other ways.
Accordingly, it would be advantageous to have a digital video encoder capable of generating chrominance data with no degradation when a VCR source is used. It would be of further advantage to eliminate the need for the digital video decoder to transmit any frequency correction data along with the data.